Conventionally, in an internal combustion engine, in order to enhance fuel economy, it is important to reduce friction in a piston. Especially in a piston ring of the internal combustion engine, it is required to enhance the quality of a pressure ring and the oil ring to reduce the friction. Also reduction of oil consumption is required as well as the reduction of the friction.
For example, the oil ring performs a scraping-off function of lubricating oil and an oil control function of controlling the consumption of the lubricating oil. In order to enhance these functions of the oil ring, a technique for thinning the axial width of the oil ring has been developed.
One example of a conventional thinned oil ring is shown in FIG. 4. In this example, an oil ring 1 has two rails 2 and 3 which are connected to each other by a column shaped web 4, and has an I-shaped cross section. In this example, the two rails 2 and 3 are formed in symmetric to each other. The oil ring 1 includes sliding projections 5 each provided with, at its tip end, a sliding surface 6 which slides with an inner wall 21 of a cylinder 20, outer side surface of sliding projections 7 forming outer side portions of the sliding projections 5, and inner side surface of sliding projections 8 forming inner side portions of the sliding projections 5. An outer peripheral groove 9 formed by connecting the rails 2 and 3 to each other by the web 4 is a groove in which oil scraped by the sliding surfaces 6 from a cylinder inner wall 21 is accommodated. The oil accommodated in the outer peripheral groove 9 passes through a large number of oil holes 16 formed in the web 4 and move toward an inner periphery of the oil ring 1.
In the oil ring having such a structure, in order to reduce the consumption of lubricating oil, an axial width of the oil ring, that is, a height “h” shown in FIG. 4 is reduced, thereby enhancing the oil control function of the oil ring. As the height “h” is smaller, the friction can be more reduced, this is advantageous for reducing the consumption of the lubricating oil. However, unlike a three-piece oil ring, the sliding surface always tries to contact flatly to a bore in the structure of a two-piece oil ring. Therefore, a contact surface formed at an initial stage is maintained for a long term.
Since a wedge effect does not contribute to the flat surface, an oil film of the sliding surface is thin, and friction becomes larger when a thin oil film is sheared off by a flat wide surface.
Since the oil ring is operated with fluid lubricant in most of regions of strokes, the velocity gradient in the oil film becomes larger when the oil film is thin, and the shearing force is increased as the velocity gradient is greater. Therefore, in a wide flat surface, friction becomes larger throughout the entire surface of a high shearing force generating region.
To avoid this, it is most important to reduce the contact width against a sliding surface.
Japanese Utility Model Publication No. 46-12405 discloses a technique in which in a piston ring having upper and lower rails whose axial widths of the piston ring are thick, the sliding surface of the piston ring is formed into a barrel like shape in order to prevent the seizing of the piston ring which may occur due to excessive pressure applied to an edge of the piston ring which comes into contact with an inner wall of the cylinder, when an inconvenience such as swinging, a lateral oscillation of the piston, accompanied by the vertical movement of the piston.
Furthermore, Japanese Patent Application Laid-open No. 2002-71021 discloses an oil ring having excellent oil consumption. In this publication, upper and lower rails of the oil ring are respectively provided with first tapered surface and second tapered surface, and an angle between the lower rail side sliding surface and the second tapered surface is set larger than an angle of the upper rail. Thus, the oil film is increased when the ring moves upward and is reduced when the ring moves downward, thereby reducing the oil consumption. However, this oil ring has a problem that it is necessary to form the upper and lower tapers such that they become different from each other and to judge the direction when the oil ring is assembled, and the work operation becomes complicated.